Background
Hypoxia is a condition in which of low or depleted dissolved oxygen concentrations are observed in the waters of Long Island Sound (LIS). Hypoxia impacts up to half of the LIS waters each summer. The primary cause of hypoxia is excess nitrogen from human sources. These sources include sewage treatment plant discharges, storm water runoff and atmospheric deposition.
Dissolved oxygen (DO) levels below 3.0 mg/L are considered hypoxic in LIS. Hypoxic conditions cause impairment and in some cases death to aquatic life. Some studies have found DO can become limiting below 4.8 mg/L for sensitive fish species while more tolerant species are not affected until DO falls below 2.0 mg/L (Simpson et al. 1995, 1996).
Since 1991, the Connecticut Department of Energy and Environmental Protection (CT DEEP) and the Interstate Environmental Commission (IEC) have conducted an intensive water quality monitoring program on LIS. The program is funded through a grant from the U.S. Environmental Protection Agency’s (EPA) Long Island Sound Partnership (LIS Partnership). In 2003, the Long Island Sound Integrated Coastal Observing System (LISICOS) consisting of a series of buoys equipped with continuous water quality, current, and meteorological monitoring sensors was deployed across LIS to increase the temporal resolution of collected data. Data from the surveys are used to quantify and identify annual trends and differences in water quality parameters relevant to hypoxia, in particular nutrients, temperature, and chlorophyll. These data are also used to evaluate the effectiveness of the LIS management programs’ efforts to reduce anthropogenic nitrogen inputs. Nitrogen is a primary contributor to the excessive algae growth that leads to hypoxia in LIS.
During the summer (June - September), surveys across LIS (Figure 1) are conducted at bi-weekly intervals to better define the areal extent and duration of hypoxia. During these surveys stations are sampled for in-situ parameters including dissolved oxygen, temperature, pH, and salinity.
Figure 1: Interactive map of LIS survey stations. Click on each station to find information on sampling index period for that station. UCONN LISICOS Buoy Stations in Norwalk Harbor, Thames River, and the Eastern Sound have been discontinued.
During the summer of 2025, CT DEEP conducted seven surveys across LIS between June 2 and August 28. IEC conducted twelve surveys in western LIS between June 17 and September 9. Hypoxic conditions were documented during two CT DEEP surveys and seven IEC surveys.
| Cruise | Start Date | End Date | Number of Stations Sampled | Number of Hypoxic Stations | Hypoxic Area (mi2) | Minimum DO | Station Where Minimum DO Occurred |
|---|---|---|---|---|---|---|---|
| WQJUN25 | 6/2/25 | 6/4/25 | 17 | 0 | 0 | 7.46 | A4 |
| HYJUN25 | 6/17/25 | 6/18/25 | 30 | 0 | 0 | 6.54 | E1 |
| IEC RUN #1 | 6/24/25 | 6/24/25 | 23 | 0 | NC | 3.65 | 8-403 |
| WQJUL25 | 6/30/25 | 7/2/25 | 40 | 0 | 0 | 4.50 | A4 |
| IEC RUN #2 | 7/1/25 | 7/1/25 | 24 | 0 | NC | 3.33 | 9-413 |
| IEC RUN #3 | 7/8/25 | 7/8/25 | 24 | 4 | NC | 0.95 | 9-413 |
| HYJUL25 | 7/14/25 | 7/16/25 | 39 | 1 | 18.07 | 2.85 | A4 |
| IEC RUN #4 | 7/18/25 | 7/18/25 | 24 | 5 | NC | 2.52 | H-C1 |
| IEC RUN #5 | 7/24/25 | 7/24/25 | 24 | 2 | NC | 2.35 | A5 |
| IEC RUN #6 | 7/29/25 | 7/29/25 | 24 | 6 | NC | 2.38 | H-C1 |
| WQAUG25 | 7/29/25 | 7/31/25 | 37 | 1 | 18.34 | 2.47 | A4 |
| IEC RUN #7 | 8/5/25 | 8/5/25 | 24 | 8 | NC | 2.50 | A3 |
| IEC RUN #8 | 8/12/25 | 8/12/25 | 24 | 3 | NC | 2.43 | B3M |
| HYAUG25 | 8/11/25 | 8/13/25 | 39 | 0 | 0 | 3.05 | B3M |
| IEC RUN #9 | 8/19/25 | 8/19/25 | 24 | 1* | NC | 1.91 | A3 |
| IEC RUN #10 | 8/26/25 | 8/26/25 | 24 | 0 | NC | 4.34 | H-B |
| WQSEP25 | 8/25/25 | 8/28/25 | 40 | 0 | 0 | 4.60 | A4 |
| IEC RUN #11 | 9/2/25 | 9/2/25 | NA | 0 | 0 | 3.30 | H-B |
| HYSEP25 | No Survey Conducted DO>3.0 mg/L | NA | NA | NA | NA | NA | NA |
| IEC RUN #12 | 9/18/2025 |
|
NA | 0 | NC | 3.57 | H-B |
Weather Summary
The Summer of 2025, defined as the months of June, July, and August, ranked among the hottest on record for Islip, NY (4th hottest) and Bridgeport, CT (13th); while Hartford, CT was close to normal. Air temperatures over the period at Bridgeport averaged 73.9ºF which was 0.6 degrees above the normal average of 73.3ºF. At Islip, summer 2025 air temperatures averaged 74.5ºF which was 1.9ºF above normal. Hartford summer air temperatures averaged 72.2ºF in 2025.
Figure 2: 2025 Hartford, CT Air Temperature Departures from Normal. Source: NowData- NOAA Online Weather Data, Daily data for a month https://www.weather.gov/wrh/Climate?wfo=box
Precipitation across the Long Island Sound region varied over the summer season. In Hartford, May and July was wetter than normal while June and August were drier than normal. This lead to the summer of 2025 being close to average for Hartford with 12.39 inches of precipitation falling compared to the normal 12.66 inches. Islip also received close to average precipitation (8.05 inches in 2025 compared to the average 11.50 inches). Bridgeport, however, only received 21% of the normal amount of precipitation (2.34 inches versus 11.07 normally) making 2025 one of the driest on record.
Figure 3: 2025 Bridgeport, CT precipitation departures from normal. Source: NowData- NOAA Online Weather Data, Daily data for a month https://www.weather.gov/wrh/Climate?wfo=okx
Wilson et. al., (2008) postulated that wind direction can impact hypoxia in LIS, with winds approaching 203ºT producing maximum stratification and hypoxia, while winds out of the northeast contribute to decreased hypoxic area. Duval et.al., (2023) suggest, after examining continuous buoy data, that “horizontal advection of DO by tides from the Upper East River into Western LIS” is responsible for modulating hypoxia.
Performing full scale statistical analyses of the 2025 wind and tidal current data are outside the scope of this CTDEEP report.The following plots were created by the ACIS Wind ROSES Wind Plotting Application on the Northeast Regional Climate Center website and are presented to show that dominant wind direction varies from station to station across the Sound’s coastal watershed.
Figure 4: Windrose graphs of wind speed and direction from four regional airports around Long Island Sound during the summer of 2025.
The LISICOS Buoy are equipped with meteorological sensors that record air temperature, wind direction, and wind speed. Wind data from the Western LIS Buoy during the summer months are shown below in a windrose plot created using R software by CTDEEP staff. Wind data from the Execution Rocks Buoy were not available. The pattern appears to be more similar to Bridgeport and Islip than LaGuardia and Westchester County with approximately 18% of the wind counts coming out of the southwest; the direction favorable for maximum stratification and hypoxia. However, it also shows about 15% out of the east; the direction that alleviates hypoxia.
Figure 5: Windrose graph of wind speed and direction from Western Long Island Sound LISICOS Buoy during the summer of 2025
Water Temperature
Long Island Sound water temperatures have generally been increasing over the 30+ years of the survey (Howell and Auster, 2012; Rice and Stewart, 2013; O’Donnell, et. al., 2020; and Georgas, et. al., 2016). This year, average water temperatures continued to decrease from a five-year high in 2020. The interactive graph below shows the yearly average water temperature across Long Island Sound.
Figure 6: Interactive map of average annual surface and bottom water temperatures measured in Long Island Sound. Scroll over the data points to see values. Use the slider to zoom to a particular time period.
Figure 7: Interactive map of surface and bottom water temperatures measured during CT DEEP water quality cruises at Station A4 in Western Long Island Sound. Scroll over the data points to see values. Use the slider to zoom to a particular time period.
Figure 8: Interactive map of surface and bottom water temperatures measured during CT DEEP water quality cruises at Station E1 in Central Long Island Sound. Scroll over the data points to see values. Use the slider to zoom to a particular time period.
Figure 9: Interactive map of surface and bottom water temperatures measured during CT DEEP water quality cruises at Station M3 in Eastern Long Island Sound. Scroll over the data points to see values. Use the slider to zoom to a particular time period.
Maximum Hypoxic Area
CT DEEP utilizes survey data and geographic information system (GIS) software to interpolate bottom water dissolved oxygen concentrations across LIS and estimate the area effected by hypoxia for each survey. Changes in the maximum hypoxic area over the LIS sampling period of record can be seen in this video. 2025 CT DEEP dissolved oxygen maps are available on the DEEP Monitoring Maps webpage. Maps created by the IEC for the far Western Sound 2025 season can be seen in this video.
In 2025, the maximum area of Long Island Sound with bottom water dissolved oxygen (DO) concentrations below 3.0 mg/L was 47.5 sq. km (18.34 sq. mi) and occurred during the WQAUG25 survey conducted 29 - 31 July 2025 (Figure 3). During this survey one station had concentrations below 3.0 mg/L, two stations had concentrations between 3 and 3.5 mg/L, and 23 stations had concentrations between 3.5and 4.8 mg/L. The lowest dissolved oxygen concentration recorded during the survey was 2.47 mg/L at Station A4. This video illustrates the progression of hypoxia over the 2025 season.
Figure 10: LIS Dissolved Oxygen concentrations recorded during August 2025 survey
The LIS Partnership utilizes a five-year rolling average (Figure 11) to determine progress towards meeting the management target of reducing the area of LIS bottom water exhibiting hypoxic conditions. The five-year rolling average is used to assess trends because conditions in any given year could be impacted by variable factors, such as extreme weather conditions. The years 1987-1999 are used as a baseline because they represent the beginning of LIS Partnership’s water quality monitoring program prior to the LIS Total Maximum Daily Load (TMDL) developed in 2000 targeting a reduction in nitrogen loads to LIS. Read more about the hypoxia extent ecosystem target established by the LIS Partnership.
Figure 11: This interactive graph shows the maximum area of hypoxia in square miles (blue bars), duration of hypoxic conditions in days (black circles), and the five-year rolling average area in square miles (green line).
Duration
The duration of hypoxia refers to the estimated length of time that hypoxic conditions persist in the bottom waters of Long Island Sound. Duration is estimated based on dissolved oxygen concentration readings taken at three water quality monitoring stations in the Western Sound - A3, A4, and B3.
The 2025 hypoxic event lasted an estimated 40 days, beginning on July 14, 2025 and ending on August 22, 2025 (Figure 12).
The earliest onset of hypoxia (based on CT DEEP data only) occurred on June 25, 2002 and the latest end date was September 20, 2005. The average duration over the 33-year time series is 52 days. The longest hypoxic event was 79 days during 2008. The shortest hypoxic event was 26 days in 2017.
Figure 12: Timing and Duration of Hypoxia in Long Island Sound 1991 - 2025 based on DEEP survey results.
Table 2 displays the onset, duration, and end of the hypoxic events from 1991 through 2025 based on CT DEEP data only. Using the LIS Partnership dissolved oxygen standard of 3.0 mg/L, the average date of onset was July 12 (+/-9 days), the average end date was September 3 (+/-11 days), and the average duration was 52 days (+/-13 days).
The earliest onset of hypoxia occurred on June 25, 2002, and the latest end date occurred on September 20, 2005.
The maximum area of hypoxia was 393 square miles and occurred in 1994. The longest hypoxic event occurred in 2008 and lasted 79 days. The shortest hypoxic event occurred in 2017 and lasted 26 days.
In 2014, 2016, 2017, 2018, and 2020 there were clear periods where the DO concentration rose above the 3.0 mg/L threshold in the early/middle part of August before dipping again during late August and early September.
| Year | Estimated Start Date | Estimated End Date | Duration (days) | Maximum Area (mi2) |
|---|---|---|---|---|
| 1991 | 07-19 | 08-28 | 41 | 122 |
| 1992 | 07-07 | 08-30 | 55 | 80 |
| 1993 | 07-09 | 09-10 | 64 | 202 |
| 1994 | 07-01 | 09-06 | 68 | 393 |
| 1995 | 07-12 | 08-16 | 35 | 305 |
| 1996 | 08-10 | 09-12 | 34 | 220 |
| 1997 | 07-27 | 09-12 | 48 | 30 |
| 1998 | 07-05 | 09-16 | 73 | 168 |
| 1999 | 07-02 | 08-21 | 51 | 121 |
| 2000 | 07-02 | 08-06 | 35 | 173 |
| 2001 | 07-10 | 09-14 | 66 | 133 |
| 2002 | 06-25 | 08-28 | 65 | 130 |
| 2003 | 07-05 | 09-03 | 61 | 345 |
| 2004 | 07-20 | 09-12 | 55 | 202 |
| 2005 | 07-14 | 09-20 | 69 | 177 |
| 2006 | 07-06 | 08-27 | 53 | 199 |
| 2007 | 07-16 | 09-11 | 58 | 162 |
| 2008 | 07-03 | 09-19 | 79 | 180 |
| 2009 | 07-19 | 09-01 | 45 | 169.1 |
| 2010 | 07-05 | 08-13 | 40 | 101.1 |
| 2011 | 07-06 | 08-28 | 54 | 130 |
| 2012 | 07-10 | 09-10 | 63 | 288.5 |
| 2013 | 07-08 | 09-07 | 62 | 80.7 |
| 2014* | 07-24 | 09-09 | 35 | 87.1 |
| 2015 | 07-16 | 09-10 | 57 | 38 |
| 2016* | 07-08 | 09-03 | 51 | 197.5 |
| 2017* | 07-18 | 08-29 | 26 | 70 |
| 2018* | 07-30 | 09-08 | 35 | 51.6 |
| 2019 | 07-12 | 08-28 | 48 | 89.4 |
| 2020* | 07-07 | 09-10 | 43 | 132.5 |
| 2021 | 07-23 | 09-07 | 47 | 142 |
| 2022 | 07-10 | 09-05 | 58 | 86.6 |
| 2023 | 07-12 | 08-22 | 42 | 126.8 |
| 2024 | 07-11 | 08-28 | 38 | 43.4 |
| 2025 | 07-14 | 08-22 | 40 | 18.34 |
| Average | 07-12 | 09-03 | 51.3 | 148.4 |
| Deviation | ±9 days | ±11 days | ±13 days | ± 86.98 mi2 |
The University of Connecticut Long Island Sound Integrated Coastal Observing System (LISICOS) utilized continuously collected dissolved oxygen data from the Execution Rocks buoy (Figure 13) to estimate there were 46.97 days with concentrations below or equal to 3.0 mg/L. There were 28.86 days with DO concentrations less than or equal to 2.0 mg/L and 1.33 days with concentrations below or equal to 1.0 mg/L. The lowest DO concentration recorded by the buoy was 0.54 mg/L on July 21.
Figure 13: Execution Rocks LISICOS Buoy Continuous DO Data June 2025 - October 2025
Hypoxic Volume
Hypoxic Volume is a measure of the vertical extent or thickness of hypoxia. Hypoxic volume is an important measure for aquatic life uses because fish and crustaceans move throughout the water column.
In 2019, CT DEEP and the O’Donnell lab at the University of Connecticut Marine Sciences Department undertook a project to develop a tool to calculate the hypoxic volume of Long Island Sound. The tool is available to the public and allows users to obtain area and volume estimates on the fly from any survey from 1991-present. The tool utilizes CT DEEP and IEC data.
The maximum volume of water with concentrations below 3.0 mg/L occurred during the WQAUG25 survey and was 0.13 km3 (0.03 mi3). Table 3 compares hypoxic area and volume by survey over the 2025 season. Figure 14 shows a time series of the maximum hypoxic volume from 1991 to 2025.
|
Area (sq km)
|
Volume (cubic km)
|
|||||
|---|---|---|---|---|---|---|
| CRUISE | 2 mg/L | 3 mg/L | 4.8 mg/L | 2 mg/L | 3 mg/L | 4.8 mg/L |
| WQJUN25 | 0 | 0 | 0 | 0 | 0.00 | 0.00 |
| HYJUN25 | 0 | 0 | 0 | 0 | 0.00 | 0.00 |
| WQJUL25 | 0 | 0 | 53 | 0 | 0.00 | 0.04 |
| HYJUL25 | 0 | 49 | 311 | 0 | 0.02 | 1.13 |
| WQAUG25 | 0 | 66 | 1220 | 0 | 0.13 | 9.82 |
| HYAUG25 | 0 | 0 | 1373 | 0 | 0.00 | 11.88 |
| WQSEP25 | 0 | 0 | 62 | 0 | 0.00 | 0.02 |
Figure 14: The interactive graph plots the maximum hypoxic volume over time. Initial analyses shows strong evidence that the hypoxic volume in LIS is substantially smaller between 2015 and 2019 than 20 years earlier (O’Donnell, et al., 2020).
Frequency of Hypoxia
Figure 15 illustrates how frequently the areas of Long Island Sound experience hypoxia. The colors represent the percentage of years in which hypoxic conditions have occurred in the bottom waters of Long Island Sound. The westernmost areas have experienced hypoxia almost every year since monitoring began.
The bar graph (Figure 16) provides a deeper dive into the data (percentages) used to create the frequency map. The colors of the bars correspond to the map legend.
Figure 15: The frequency of hypoxia in Long Island Sound Bottom Waters from 1994 - 2025
Figure 16: The percentage of time a station has been hypoxic over the period of survey record
Hypoxia and Coastal Acidification
In Long Island Sound, hypoxia co-occurs with coastal acidification. Coastal acidification is a term that describes the decline in pH and other changes in water chemistry over time in waters close to shore. Increased atmospheric carbon dioxide from burning fossil fuels, freshwater input, excess nutrient runoff, and coastal upwelling can lead to acidification. Excess carbon dioxide is absorbed in the ocean, resulting in the formation of carbonic acid and the lowering of pH. These conditions develop both seasonally and diurnally in response to increasing temperatures and eutrophication, respectively. To learn more, visit the Northeast Coastal Acidification Network and National Oceanic and Atmospheric Administration Ocean Acidification Program websites.
Marine life and coastal communities are impacted by coastal acidification in a number of ways. The increase in carbonic acid results in a decrease in carbonate ions available for calcifying organisms like oysters and clams to use to make and maintain their shells. Fish species like flounder and squid exposed to higher than normal acidic conditions can show reduced larval growth and survival. Additional research is needed to better understand the effects of coastal acidification on species, as well as the effects of coastal acidication in combinations with other stressors such as hypoxia.
In 2023, CTDEEP, IEC, and USGS began an ocean acidification monitoring program around LIS (Figure 17).
Figure 17: Coastal acidification monitoring stations around Long Island Sound
To characterize the carbonate system total alkalinity, dissolved inorganic carbon, and spectrophotometric pH samples are collected monthly from ten (10) stations. CTDEEP samples are collected from 2 meters below the surface (surface) and 5 meters above the sediment (bottom). The sampling builds upon earlier studies by the Vlahos Lab at the University of Connecticut. Preliminary data from 2023-2025 are presented below in an interactive dashboard.
Aragonite is one of the more soluble forms of calcium carbonate and is widely used by shellfish, coral, crustaceans, and even some species of phytoplankton to build shells and skeletons. When the aragonite saturation state is above 3, the survival and reproduction rates of marine calcifers are better/normal. When levels fall below 3, these organisms become stressed. When aragonite saturation states fall below 1, shells begin to dissolve. Aragonite saturation states derived from spec pH and DIC using CO2SYS are included in the dashboard for 2023 and 2024; 2025 data were not yet available. Calculated LIS aragonite saturation states in these two years were below 1.5 with the lowest values found during Spring. These values were generally consistent with those calculated by Gledhill et. al. (2015). (See the regional conditions map.)
Figure 18: Interactive Coastal acidification Data Dashboard. Dashboard created by Kaleb Boudreaux after concept created by Goes Lab at Columbia University
Data Downloads
Data from all CT DEEP cruises can be downloaded from the UCONN ERDDAP and through the Water Quality Portal (Organization ID: CT_DEP01_WQX). All samples are collected and analyzed under EPA-approved Quality Assurance Project Plans.
Data from IEC cruises can be downloaded through the water quality portal (Organization ID:31ISC2RS_WQX). All IEC samples are collected and analyzed under EPA-approved Quality Assurance Plans.
In addition click the buttons below to download data utilized in the visualizations produced for the 2025 Hypoxia Season Summary.
Contact Information
For more information on the Long Island Sound Water Quality Monitoring Program visit:
Katie O’Brien-Clayton, CT DEEP
katie.obrien-clayton@ct.gov
Acknowledgements:
Cover photo: Carriel Cataldi, LIS Water Quality Monitoring Program Staff
This Project was funded by the United States Environmental Protection Agency, Long Island Sound Partnership through grant funds administered by CT DEEP and IEC.
References
Duvall, M.S., Hagy, J.D. 3rd, Ammerman, J.W., Tedesco, M.A. High-frequency dissolved oxygen dynamics in an urban estuary, the Long Island Sound. Estuaries Coast. 2023;47:415-430. doi:10.1007/s12237-023-01278-8.
Georgas, Nickitas, Lun Yin, Yu Jiang, Yifan Wang, Penelope Howell, Vincent Saba, Justin Schulte, Philip Orton, and Bin Wen. 2016. An Open-Access, Multi-Decadal, Three-Dimensional,Hydrodynamic Hindcast Dataset for the Long Island Sound and New York/New Jersey Harbor Estuaries. Journal of Marine Science and Engineering. 4(3):48. doi:10.3390/jmse4030048. www.mdpi.com/journal/jmse.
Howell, Penelope and Peter J. Auster. 2012. Phase Shift in an Estuarine Finfish Community Associated with Warming Temperatures. Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 4:481–495, 2012. DOI: 10.1080/19425120.2012.685144.
O’Donnell, J., Fake, T., and J. O’Donnell. 2020. Computing the Hypoxic Volume of Long Island Sound- Final Report, September 12, 2020. University of Connecticut, Department of Marine Sciences, Groton, CT. Prepared for the Connecticut Department of Energy and Environmental Protection and the Long Island Sound Study
Rice, Edward and Gillian Stewart. 2013. Analysis of interdecadal trends in chlorophyll and temperature in the Central Basin of Long Island Sound. Estuarine, Coastal and Shelf Science. 128: 64-75, ISSN 0272-7714, https://doi.org/10.1016/j.ecss.2013.05.002.
Simpson, D. G., Gottschall, K., and Johnson, M. 1995. Cooperative interagency resource assessment (Job 5). In: A study of marine recreational fisheries in Connecticut, CT DEP Marine Fisheries Office, PO Box 719, Old Lyme, CT 06371, p. 87-135.
Simpson, D.G., Gottschall, K., and Johnson, M. 1996. Cooperative interagency resource assessment (Job 5). In: A study of marine recreational fisheries in Connecticut, CT DEP Marine Fisheries Office, PO Box 719, Old Lyme, CT 06371, p. 99-122.
Wilson, Robert E., Swanson, R. Lawrence, and Crowley, Heather A. 2008. Perspectives on long-term variations in hypoxic conditions in western Long Island Sound. J. Geophys. Res. 113, C12011. doi: 10.1029/2007JC00469.